How to fill ngon with faces?
Ok so I'm under pressure and I feel the need to edit.

I gave you the method to print a grid topology on your mesh because that's what you asked. But, unless you have good reason to use this kind of topology, it is actually not very good. You should better use this kind of topology (coming from a previous question of BSE), which use only quads, can easily be bent and modified with new edge loops, etc. In your case, if you want to keep your current mesh, you can first simplify it with some Checker Deselect and Dissolve Vertices, then fill with F and cut some edges with the knife:



Former answer:

What you can do is first fill your shape with a single face (F), create a plane, subdivide it to have a grid, delete its faces only, back to Object mode put it over the leaf object, Shift select the leaf, stay in top view, go in Edit mode, then Mesh > Knife Project. It will print the square grid on the leaf mesh.



Once filled, you could also give your face several knife cuts, it will create less faces.